1. Field of the Invention
The present invention relates to a treatment method and a treatment system for decomposing and making harmless efficiently chlorinated organic compounds such as dioxins contained in exhaust gas from cement manufacturing equipment that uses incineration ash as part of the cement raw materials.
2. Description of the Related Art
In recent years, there have been an increase in occasions of using waste materials containing dioxins such as municipal refuse, soot (i.e. fly ash) and combustion residue (i.e. bottom ash) generated upon incinerating municipal refuse, sewage sludge, paper-making sludge, and molding sand as raw materials in cement manufacture.
Among these waste materials, municipal refuse, soot (i.e. fly ash) generated upon incinerating municipal refuse, sewage sludge, paper-making sludge and so on are charged into a pre-calciner or a rotary kiln that constitutes part of the cement manufacturing equipment, and hence are put under conditions under which the dioxins can be decomposed.
On the other hand, there are also some kinds of waste materials that are charged into a suspension preheater together with ordinary cement raw materials. Examples of such waste materials include coal ash, molding sand, combustion residue (i.e. bottom ash) generated upon incinerating municipal refuse. Dioxins contained in these waste materials may not be completely decomposed, and some dioxins may vaporize and remain in the exhaust gas.
Dioxins that are not decomposed and remain in a very small quantity in the exhaust gas are adsorbed onto the surface of dust contained in the exhaust gas as the, temperature of the exhaust gas drops. The dust on which the dioxins have been adsorbed is then separated from the exhaust gas and collected by equipment installed in the gas flue such as a dust collector (e.g. electrostatic precipitator), a drier and a mill.
In this way, most of the dioxins generated during cement manufacture are decomposed or collected, and are not discharged out of the exhaust gas treating system.
However, dioxins that have not been adsorbed on the dust, and gaseous dioxins, are discharged into the atmosphere together with the exhaust gas, albeit in a very small quantity. The discharge concentration of these dioxins is sufficiently low, but it is desirable to continue striving for further reductions.
Under the circumstances, various arts for decomposing and making harmless the dioxins contained in exhaust gas of cement manufacturing equipmentes have been proposed.
For example, Japanese Patent Application Laid-open No. 2000-16844 discloses a method in which, when cement is manufactured by using incineration ash from municipal refuse or industrial waste material as part of the raw materials, kiln dust containing dioxins that have not yet been decomposed is collected, the collected kiln dust is heated to a temperature above the boiling points of the dioxins, and the gas containing the vaporized dioxins is introduced into a rotary kiln for clinker firing, whereby the dioxins are decomposed by the heat of the cement firing.
Also, Japanese Patent Application Laid-open No. H11 (1999)-246247 discloses a cement manufacturing equipment that is provided with firing and cooling equipment that fires raw materials including municipal refuse incineration ash or the like to produce cement clinker, exhaust gas treatment equipment that rapidly cools the exhaust gas generated from the firing and cooling equipment to suppress the resynthesis of dioxins and thus make the exhaust gas harmless, dioxin decomposing equipment that collects dust generated from the firing and cooling equipment and pyrolyzes the dioxins in the dust, and so on.
In the arts disclosed in Japanese Patent Application Laid-open No. 2000-16844 and Japanese Patent Application Laid-open No. H11 (1999)-246247 described above, dioxins contained in the dust in exhaust gas of the cement manufacturing equipment are treated using independent heating equipment provided separately to the equipment originally involved in the cement manufacture such as a rotary kiln. It would be convenient if dioxins in exhaust gas of cement manufacturing equipment could be treated efficiently without providing special heating equipment for treating the dioxins.